Typical commercial reproduction apparatus include electrostatographic process copier-duplicators or printers, inkjet printers, and thermal printers. With such reproduction apparatus, pigmented marking particles, ink, or dye material (hereinafter referred to commonly as marking or toner particles) are utilized to develop an electrostatic image, of information to be reproduced, on a dielectric (charge retentive) member for transfer to a receiver member, or directly onto a receiver member. The receiver member bearing the marking particle image is transported through a fuser device where the image is fixed (fused) to the receiver member, for example, by heat and pressure to form a permanent reproduction thereon.
A primary charging device is typically used to uniformly charge a dielectric member before the dielectric member is exposed to an imaging light pattern. The primary charging device may be for example a corona charging device including several members, such as one or more parallel thin wires to which high voltage is applied, a housing partially surrounding the wires and open in a direction facing a dielectric member surface, and an electrically biased grid. A conductive (metallic) housing is used for DC charging (i.e., applied high voltage is DC), and an insulating (plastic) housing is typically used for AC charging (i.e., applied high voltage is AC). A grid includes a metallic screen or mesh, mounted between the corona wire(s) and the dielectric member, and is DC-biased for both DC and AC charging. Use of a grid improves control of the voltage that a primary charger imparts to the dielectric member. Use of a grid also gives a resultant dielectric member voltage uniformity that is generally better than without a grid.
When using a DC charger having high voltage DC applied to the corona wire(s), if the residence time of a moving dielectric member surface passing under a gridded charger is long compared to a characteristic time constant given by the product of the effective charging resistance and the capacitance of the dielectric member under the charger, the voltage on the dielectric member will asymptotically approach a cut-off voltage equal to the DC grid bias plus an overshoot voltage determined by grid transparency, grid/dielectric member spacing and corona voltage. For tight grids (relatively low transparency) the cut-off of the charging current is very close to the grid bias; that is, the overshoot is small. Conversely, for open grids (relatively high transparency) the overshoot can be significant. For a typical grid, the overshoot can be in the range 100-200 volts, depending on the grid to dielectric member spacing, with smaller overshoots for larger spacings.
For an AC charger in which a waveform comprising high voltage AC plus low voltage DC is applied to the corona wire(s), the cut-off voltage is generally close to the grid bias, and is only weakly dependent on the grid transparency. The actual cut-off voltage is determined by the relative efficiencies of negative and positive corona emissions during the negative and positive AC voltage excursions. Moreover, a high duty cycle trapezoidal AC waveform can be used, as disclosed in U.S. Pat. No. 5,642,254 (issued Jun. 24, 1997, in the names of Benwood et al). In this patent, the cut-off voltage is also dependent on duty cycle, and the cut-off voltage steadily approaches a DC value if duty cycle is steadily increased from 50% (conventional AC) to 100% (DC).
Presently, a variety of gridded chargers are used in typical reproduction apparatus engines. Examples of grid designs include a continuous wire filament wound back and forth across a charger opening, grids (typically photoetched) mainly composed of thin parallel members that run parallel to or at an angle to the corona wire(s), and hexagonal opening mesh pattern grids. These different types of grids are applied in various types of corona chargers, for example single or multiple corona wire chargers, pin coronode chargers, chargers with insulating or conducting housings, and chargers that use AC or DC corona voltage. There are grids that are planar and grids that are curved to be concentric with a drum dielectric member.
One exemplary family of reproduction apparatus (the Eastman Kodak IS 110.TM. and Ektaprint 3100.TM.) uses a primary charger that has three corona wires powered by an AC trapezoidal voltage waveform with a DC offset, an insulating housing, and a planar tensioned grid comprised mainly of thin members that run parallel to the corona wires. The percent coverage of the grid varies in a direction perpendicular to the axis of the thin grid members (i.e., in the direction of motion of the dielectric member). The "upstream" side of the grid (the first to charge the moving dielectric member) has a percent coverage of 14.2% (transparency 85.8%), and the percent coverage increases gradually towards the "downstream" side of the grid to a percent coverage of 16.3% (transparency 83.7%). A varying coverage grid design such as this is termed "aperiodic." The aperiodicity is clearly very small for the primary charger grids; i.e., the transparency is reduced by only 2.4% from the upstream edge to the downstream edge.
In U.S. Pat. No. 3,527,941 (issued in 1970, in the names of Culhane et al), there is described the use of an aperiodic grid for primary charging. The grid includes thin parallel members whose spacing is largest on the upstream side and decreases towards the downstream side. The charger also includes a grounded conducting housing. While no quantitative range of preferred aperiodicity is mentioned, it is disclosed that the spacing of the grid members is "very great" on the upstream side. The stated advantage is to give a more rapid charge than is possible with aperiodic grid. No specific reference is made in this patent as to whether this patent is directed to DC or AC charging, but it inferentially refers to DC charging only. This can be seen in column 3, lines 29-31, which states that "where there is a high leakage, the dielectric member will tend to be charged to the potential on the corona wires". Inasmuch as the time-averaged potential from the purely AC component of an AC waveform applied to corona wires is zero, the aforementioned quote makes no sense unless it refers to DC charging. Furthermore, since the time-averaged potential of an AC waveform having a DC offset is equal to the DC offset itself, then the DC offset would have to be impracticably large to correspond to the specifications of this patent. Finally, the patent predates the usage of AC primary charging technology, so that references therein to high potentials applied to corona wires implicitly refer to DC, rather than AC, high potentials.
In U.S. Pat. No. 5,025,155 (issued Jun. 18, 1991, in the name of Hattori), there is described the use of a grid on a DC charger that is positioned so that the grid directly under the downstream-most wire is closer to the dielectric member than the grid under the upstream wire(s). In this patent (see particularly column 6, lines 1617, FIG. 5 and FIG. 8), the grid in at least one embodiment comprises two sections, with the upstream section being more transparent than the downstream section, the downstream section being also closer to the dielectric member drum. However, the patent subsequently recites that the first section (upstream) has finer openings than the downstream section. The stated advantage is that a given dielectric member voltage can be obtained at a lower corona voltage than for a standardly located charger with a constant transparency grid.
U.S. Pat. No. 4,386,837 (issued Jun. 7, 1983, in the name of Ando) discloses the use of two sequential DC chargers (e.g., chargers #1 and #2) of different polarities having a common grid potential. The grid potential is opposite in polarity to a pre-existing voltage on a dielectric member drum on the upstream side of both chargers. Charger #1 reverses the pre-existing voltage and charges the dielectric member film member to a voltage of higher magnitude but of the same polarity as the grid. Charger #2 reduces this voltage magnitude but does not reverse it, producing an exit voltage on the dielectric member drum that is close to the grid potential. In one modification, the grid of each of the chargers #1 and #2 becomes gradually less transparent in the direction of rotation of the drum, with the stated advantage being that charging is more rapid at the entrance to each charger and less rapid but more controlled in uniformity at the exit from each charger. The stated result is a uniform charging to an exit voltage close to that of the grid potential, and of the same polarity. This patent does not disclose preferred ranges of aperiodicity for either charger, nor is any uniformity improvement produced by the invention quantified.
In U.S. Pat. No. 3,797,927 (issued Mar. 19, 1974, in the names of Takahashi et al), there is disclosed a mechanism for producing a latent image on a dielectric member involving simultaneous charge and expose of the dielectric member using a gridded charger, with the distance between parallel grid wires decreasing in the direction of motion of the dielectric member and the stated advantage (column 5, lines 35-36) of "gradualization of the equalization of the surface charges". A DC simultaneous charge and expose device is disclosed (column 5, lines 33-35, FIG. 3a') as well as an AC device (column 6, lines 14-16, FIG. 4a'). This patent does not disclose preferred ranges of aperiodicity for either charger, nor is any uniformity improvement produced by the invention quantified.
U.S. Pat. No. 4,320,956 (issued Mar. 23, 1982, in the names of Nishikawa et al) discloses, in FIG. 7b, a charger grid that is less transparent at the end portions; i.e., resulting in aperiodicity in a direction at right angles to the direction of travel of a dielectric member under the charger.
As mentioned above, a charger's resultant dielectric member voltage uniformity is generally improved by the use of a grid. However, for any corona charger design, charging uniformity tends to decline over the life of a charger due to the buildup of contamination on the corona wire members. To maintain acceptable image quality, corona wire members must be periodically replaced, which causes machine down-time and generates service costs. There is, therefore, a need to increase the running time for a charger before maintenance is required. There is also a need to improve the uniformity of charging for copiers and printers, especially for high quality color electrostatographic imaging. There is also yet a further general need to improve uniformity of charging for higher throughput speeds in copiers and printers.
These needs are especially pertinent in the context of AC charging technology. There is an ongoing commercial trend to replace prior art DC charging with AC charging, particularly for negative charging, because of ever increasing demands for improved image quality. It is well known in the art that AC negative charging is much superior to DC negative charging, because AC negative charging gives substantially more uniform charge laydown on a dielectric member than negative DC.